Thursday, 4 December 2014

For a change, I am planning to build a full-size classic aerobat and have ordered the plans for a HP Magic. I think the model was used in the 1981 and 1983 World Champs so (just) falls within the UKCAA date range.

I haven't seen the plans yet (except for a fuselage cross section compared with the SSII (upper) and Curare (middle)).

The official start date for this build was the 1st June 2013. That was when I started cutting parts out.

I have actually made a start on the construction. While I am still collecting parts for the wing and fuselage I have started on the tail plane. Not very exciting, but it is a start...

Both the leading and rear spar/elevator junction have false LE and TE attached. I also want to use this model to trial the use of Kevlar sailcloth sheet as a full length hinge for all the moving surfaces. To achieve this, the rear spar is laminated and a piece of Kevlar glued into the sandwich. The Kevlar takes cyano very nicely, it is far more flexible than 3 of thick Mylar hinges and far tougher. The moving surfaces will also be laminated and sandwiched up with the hinge when I am ready to add them.

The tail ribs were marked using good old fashioned carbon paper drawn through the plan and cut from some very nice 1/16" quarter grain sheet balsa.

After a delay (I have been away for nearly a week and also disrupted by building microscope boxes for a Natural History Group that I am a member of), I have managed to get the first half of the tail plane under way.

At this rate, this will be the longest construction project I have taken on! To be fair though, I have also cut out the Firewall, the false firewall and drilled the engine mount..

The tail plane is now completed and ready for a final rub down then painting:

Tailplane tips

Adding the laminated elevators to the full length Kevlar hinges

Covered with Fibreglass cloth (attached using watered down PVA) then 3 coats of water based varnish, sanding down with fine wet and dry in between coats (used dry).

Started work on the fuselage. Three splice joints to get the front , rear and tailplane mount plus 1/16" ply doublers at the front and 1/32" ply doubler under the tailplane.

Then ran into problem no.1. The front former carries the engine mount. The curve for the upper decking is correct but does not line up with the fuselage sides.

and confirmed by co-locating the former over the template for the 1/16"ply neat front former.

I have cut slots in the fuselage sides to get them to fit but this will mean that the fuselage will be a bit thin as it flows into the upper deck. Should be OK though.

I cut the next 2 formers out and got them glued into place:

I have added the triangle stock - this is not shown on the plan.

Those old UPS batteries are wonderful for keeping things square and true...

And then this morning I attached the other side..

The UPS batteries are now my datum to ensure that the fuselage sides are parallel laterally.

Finally, a shot with the tank in place. The plan suggests a slim 14oz tank. I could only find a slim 16oz tank

Finally, the first two formers are glued in adding the 1 deg downthrust. You can just see the datum line drawn down the fuselage. I have also added the 1/2" triangle supports.

A bit more progress in the past couple of days.

Got the fuselage in the jig and carefully aligned it so that there are no bananas:

Added the 1/8" ply upper decking. Took quite a while to cut this out including the hole at the front for the tuned pipe. The pipe will sit over this deck and will be covered by a Glass fibre cowl.

Added the fin post. Like the elevators, it will use Kevlar cloth for the hinge material. The post is laminated from 2 pieces of 1/4" sheet balsa. One additional benefit for the cloth is that it adds a certain amount of strength to the supporting structure.

I have used the top decking as the lateral reference - all flying surfaces will be aligned laterally and vertically from this.

However. the post isn't vertical when the longitudinal datum is horizontal. It has a degree or so of rake. The exact measurement was taken from the plan, the fuselage levelled and the rake carefully sanded.

The fin outline is laminated from 3 pieces of 3/16" sheet balsa, tapering from 50mm at the fuselage to 12mm at the top. The swoopy curves were roughly cut before the fin was glued into place.

Here is the first pass at fitting the fin outline. Most of this weekend subsequently has been spent carefully sanding it to the correct shape (or at least what I imagine the correct shape should be - there is no profile info shown on the plan).

The next task will be to get the tail plane mounted plus the fin sub-frame in place. The tail plane has +0.75deg incidence. It will be fun working out how to get that measured accurately.

Getting the +0.75deg incidence was easier than expected. I simply measured up from the datum on the plan and replicated the same measurements on the model. Fortunately, I had to take material away.. about 3mm at the LE and 2mm at the TE. Careful sanding to maintain the profile and as I got nearer to the correct fit I also checked that the tailplane was seated laterally at the correct angle.

The fuselage is clamped in the bench vice with the upper decking checked for horizontal using a spirit level. The bench is also level!

Lots of glue to hold the tailplane down and a pair of clamps making sure that the tail is pushed firmly down into the seat.

Using the 3 pin triangulation technique (see the modelflying.co.uk Tiffie thread http://www.modelflying.co.uk/forums/postings.asp?th=86400&p=10) - but using string to ensure that both tail tips are the same distance from the firewall centreline. The tailplane is also adjusted so that each pin is an equal distance above the bench (in this case exactly 50mm)

Firewall C/L....

Underside view - with the glue being pushed out...

Finally a lower view showing the carved and sanded fin outline...

Now being left to dry for at least 12 hours...

It's amazing how much work was involved cutting and fitting the four pieces of balsa that comprised the subframe for the fin (one piece is hidden). The difficulty was measuring the double taper (upwards and rearwards and getting all the pieces lined up so the outer sheeting would fit correctly.

When dry the fin sheeting was carefully cut and glued/clamped. It was very reassuring to see that the piece I cut for the left side fitted the right side perfectly..

Again - left to dry for a few hours...

I was really quite pleased with this after it had all been sanded to shape. Probably the neatest bit of building I have ever done.

I have been working on the lower front fuselage. This involves mainly getting the retracts and fuel tank plumbed in.

The u/c plate marked up and drilled. Note the half moon at the front to provide enough space to get the tuned pipe swan neck through. I can only assume that Hanno built his in permanently, I can't afford to be so extravagant

The retract in place. The front leg pivot is about 10mm further back than that on the plan - Hanno used MK retracts, mine are HK..

I have since added the lower fuel tank floor, internal and external cheeks and started carving and sanding to shape. Photo to follow...

While waiting for the glue to dry I started work on the plug for the upper deck, This will form the first stage mould, a female will be taken from this and the final deck will be moulded from that.

The mould is carved from four blocks of dried flower arranging foam glued to a 3/8 balsa floor.

Then planed - using a razor plane - to shape. (Note: this foam is far too soft - it's easily dented - blue foam is better - I used the pink variant for the cowl)

It planes very easily although it is very soft - it will crush with the slightest provocation, The dust produced is awful - you will need a mask and sand outside if you can. It needs further sanding around the font end, It will then be covered in G/F cloth and varnished/smoothed to make the plug.

This will be a new experience for me, I have never made anything like this before.

A general shot into the front wheel bay. The floor has been added and the lower cheeks have been carved and roughly sanded. I can't do any more until I fit (buy) the undercarriage legs and then fit the remainder of the block to keep the airflow as clean as possible. From this shot, it can be seen that the front former is set too low.

This shows the front end with the 1/16" ply dummy bulkhead in place. Despite being cut from the plan, its about 10mm too low. I have worked out that there is a draughting error in the plan. The former shown (and copied) - although it has the correct hole location for the exhaust and mount is actually the front section displaced up to and including the GF upper cowl. This is not where this former (F1) is shown on the plan The reality is that there is an air gap (to allow cooling air to flow through) above the fuselage but inside the top cowling. The compounded result of this error is that my fuselage is too narrow at the top and too wide at the bottom. Nothing irrecoverable though.

Started work on the front cowl. Basically, its 2 pieces of lite ply backbone and all the parts glued to it have been partly split in case i need to split the plug into 2 pieces

Adding reinforcement to try and keep it all square.

In the meantime, the shroud plug has been subject to more sanding.. Sanding this outside because the dust is awful.. I have also added more reinforcement and extensions front and rear. I'll make it too big and cut it back..

A trip to B&Q (to buy fence posts) and I spotted this - I think its the pink version of blue foam that is used in foam wing cores. If so, it's a nice manageablesize, 60mm thick, about 600mm wide and just over 1m long. Cost about £7.00 per sheet. I am using this for the foam fill for the cowl plug.

This is all the info I have to show me what the front end should look like. In addition to the plan, but there is no detailed section information.

About as much use as a chocolate fireguard.

Before I started carving then sanding

And after about 3 hours work. There are a few dings to fill, but it's 99% done.

It now needs to be sealed.

Back to finishing the foam ready for moulding:

Danny Fenton wrote:

Hi Martyn,

the finish I get on the blue foam is really nice and I do it with those foam packed sanding pads that you can get from Halfords. I tend to take the newness off by using them on balsa or ply first, then they sort of buff the foam. However the foam is not strong enough to use as is and needs a layer of glass cloth (25gsm skinning cloth) applied with EPOXY resin. Once cured brush on another coat of resin and once that is cured you can sand it with normal abrasives and apply filler/primer etc.. to get it super smooth for making a mould.

If you are making a one-off, skip the mould, just skin with perhaps a couple of coats of skinning cloth, and when hard dissolve out the foam with thinners you are left with a hard shell.

--

I visited my local G/F supplier and specialist. He thinks that with a bit more work, the cowl plus can be made in one piece as it tapers towards the nose in all directions.. We will see. I am now £50+ lighter after buying cloth, matting, tissue, resin (epoxy and polyester), release agent and gel coat.

So, still working on the plugs. Both have now been coated in resin and I am currently 'flatting' them ready for mould making. The man in the G/F shop said make sure that the casts (his words) were 100% non porous or it won't work

Some photos:

The front end of the top cowling has been extended using balsa so I have an extended flange that will be cut back. This has subsequently been covered in 60gm cloth and glassed.

I bought a Wirebender - it took 2 minutes to make this nose leg from 8g piano wire. An absolute bargain.

The original stub for the nose wheel was 5mm. 8g plus a brass tube sleeve (as suggested by Bob) was 4.9mm, Its a bit sloppy. The original stub was swaged to prevent it falling out. I put an additional brass tube collar over the sleeve brass tube and soldered the whole lot up. Because of the bend in the leg, it had to be done in situ, but it isn't going to fall out (in a hurry).

Started installing the radio gear in the fuselage. The servo tray is 1/8 birch ply with beech cross bearers and 1/4" sq hard balsa longerons.

Front nose leg in position - retracted. I had to move the retract unit forward about 10mm to get it to fit, which means that the motor for the retract is slightly in the engine bay. Nothing that can't be resolved with a bit of spruce and ply to form a cover.

The wheel extended. Note the slight caster angle bent in to get the wheel to track properly and also to fit cleanly into the bay

And another shot.

Ad here it is working..

I am still working on the plugs for the cowl and the upper deck, However, I have decided that tonight I have wet and dried them enough. I'll have a practice making a mould of something that doesn't matter if I get it wrong, possibly a tapered drinking glass or similar before I risk my precious plugs..

A bit more work on the fuselage though.

I have used goldenrods for the elevator linkages, one for each elevator. The outer tubes are terminated on a lite-ply bulkhead , the inners on a brass plate soldered to an 18swg piano wire linkage that goes to the elevator servo and back through the bulkhead. This will allow me to adjust either elevator separately - the piano wire rod prevent the plate from twisting - i.e. it moves linearly. There is also another intermediate lite-ply bulkhead half way down the fuselage to stop the rods from whipping. It looks very neat.

There is a hook that supports the rudder goldenrod outer - screwed to the throttle (middle) servo securing screw.

Because the rear fuselage is so narrow, the goldenrod outer exit underneath the LE of the tailplane. Again, to keep them stiff, they have been glued (epoxy) to a balsa fairing. This keep the outer tubes nice and stiff and hopefully, there will be no slop. Note the rudder outlet and rod - waiting for the rudder which will be a good while yet. I am a bit too clumsy to put the rudder on at this stage..

I am using these rather elegant Tornado arms for all the primary functions. They are rock solid and the clevis is actually screwed into place - through a ball race. Very smooth, but not cheap.

And the engine goes in - so I can set up the throttle linkage. The engine has a slide carb - I have never seen one before on an aero engine

Finally, a test fit for the tuned pipe. Brackets will need to be made to hold it in place.

I am not totally happy with the elevator linkage yet. There is a tiny bit if binding somewhere with full up elevator which I would like to get resolved before I put the bottom sheeting in.

As implied, a little bit of progress this week. Work and family commitments tend to get in the way, but these are more important.

The bottom sheeting has now been added. The binding problem was simply I had more throw than the elevators could handle - fixed by moving the servo linkage inwards one hole. I tend to cut my sheeting oversize and trim back when dry. Note that the grain is across the fuselage.

The other time consuming activity - I have practised making a mould before I do it for real.

I actually used a small plastic sauce pot (free with fish from Asda) - which I use for mixing glues.

The process that I followed was,

1. Plasticine on work surface and rolled out.

2. place dish onto Plasticine and remove excess around the edge using a scalpel

3. Add PVA release agent using a sponge (no brush marks) - let it dry

4. Mixed Gel coat on scales (I mixed about 50gms with 0.5gm of hardener (100:1) and apply (I actually used a spatula - a brush would have been better). Let it harden overnight

5. Apply 100gms polyester resin over 1 layer tissue, 2 layers of matting. One thing I learned was that it is much easier to use small pieces than big pieces - just keep overlapping them. I used a brush and just kept dabbing until it was all covered. Let it harden overnight

6. Pop it off the work surface - a sheet of polycarbonate 'glass'

7. Remove the Plasticine

8. Trim the flange back

9. Pop out the master - took a bit of effort but I managed to do this without damaging it..

Voila!

Quite pleased with that!

Progress has been a bit slow, a new job plus being away last weekend has slowed things down a bit.

However I have been working on the Fibreglass bits.

I have created moulds for both the cowling and the tunnel. I didn't get photos of the cowling because the camera phone managed to get resin all over itself, at which point I banned it from the workshop until it learned how to keep itself clean

The tunnel glued down on a bed of Plasticine - that in itself was an evenings work as the Plasticine was not very pliable in the cool conditions.

Laid up with 1 coat of release agent (1 evening) and allowed to dry, 1 coat of Gel (1 evening), then 1 layer of tissue and 1 layer of strand (1 evening) and allowed to harden.

I had great difficulty getting the plug out of the mould, it had attached itself in one place which resulted in destruction of the plug.. However, I am very pleased with the mould.

The same process was repeated for the cowl - which also only came out after destroying the plug - again the mould looks fine.

Next task was to make the real tunnel. 2 evenings applying wax polish then release agent (lots) followed by the gel coat. I used epoxy resin with 130gm cloth for the tunnel - doubled up at the front rear and along the edges. The resin then failed to go off, so then I read the instructions and found that I had got the mix wrong, so ripped the cloth out - it was only tacky and redid it again last night.

Overnight, the resin hardened and I tried to release it today. Once again, the tunnel and the mould were stuck together again with one area about 2" long where the gel coat stuck had to the mould. I managed to separate it and I have a light and flexible tunnel that is definitely usable although it will need a bit of work to get it ready for painting. Note that the blue release agent hasn't been cleaned off yet. The tunnel is back in the airing cupboard while the resin hardens further. Getting the tunnel out of the mould has caused a few hairline cracks in the resin and I may stiffen it up a bit using some carbon fibre tows that I have.

Outside of mould and the tunnel, It needs polishing!

A bit more progress - to be honest, I am looking forward to getting back to some proper building on this. The Flea Fli is keeping me sane at the moment.

The mould for the cowling.. before the mods

And the outside.

I am really quite pleased with this although I did have to sacrifice the master (plug) as it point blank refused to let go.. It actually broke up while trying to pull

Following Nige's advice, I added a valve through a hole roughly where the prop driver will sit and epoxied it into place.

the flange was trimmed back as much as possible and I placed a small amount of tape over the hole before I added release agent and gel coat.

More or less one layer of matting was applied and allowed to set for about 36 hours (on top of a radiator) to let it cure. I eased the sides away- and then applied some compressed air which promptly popped out the area around the prop driver face, but the air intake was stuck fast,

I eventually freed it by pouring boiling water between the cowl and the mould, another blast of air then released the air intake and a firm tug and out it popped - to a huge sigh of relief.

To be honest, it is not perfect - there are a couple of air bubbles (or remains thereof) and a couple of runs (probably the release agent was too thick), but it is usable with a bit of graft and TLC and filler..

I have also redone the tunnel using matting instead of cloth - the second attempt is much better and shows all my mistakes that I made while I made the plug..

They both now need trimming to fit, this weekends task.

I have also ordered a set of wing-ribs from Belair - they should be here at the end of next week. The wing-ribs DXF file was produced from Profili Pro...